This invention relates to underwater inspection devices, more particularly to underwater ultrasonic inspection devices for detecting fractures, inclusions, faults in welding and the like in underwater steel or other structures.
Preventive maintenance is a concept long established in land based industry. This has proven to be economically the best approach in sustaining the operation of facilities and equipment, as opposed to waiting until failure of such equipment, and then repairing broken structures and machinery. The implementation of preventive maintenance has required development of various means of establishing the operational condition of equipment and structures, and so has evolved various methods of non-destructive inspection and testing. Most structures can now be monitored throughout their service life by non-destructive tests which are extremely effective in detecting the initial breakdown of structures while such breakdowns are still easily correctable, and long before total failure of the structure. In steel structures, radiography, ultrasonics and magnetic particle inspection have developed as the primary means of inspection and testing. There is a similar need for non-destructive inspection and testing of submerged or partly submerged structures, such as the hulls of ships which cannot be dry docked, pilings, pipe lines, supports for off-shore drilling rigs, etc., which are subject to the corrosive and often violent environment of the ocean or other bodies of water.
Because of the criticality of maintaining submerged or partly submerged structures in functional condition, some development has been made in structural inspection and testing for such structures. This development has primarily evolved along the concept of utilization of the same testing and inspection means developed for land based use, and duplication of the land base habitat for which such devices are designed. This is accomplished either by removing the structure from the body of water and examination of the removed structure in a dry environment, or by constructing a dry habitat around the structure while submerged or partly submerged, transporting the testing equipment to the habitat, and performing the inspection or test as it has been performed on dry land. Both of these methods have substantial disadvantages. Both are costly and time consuming. Accordingly, some efforts have been expended in developing specialized test equipment which is portable and manageable in the underwater environment, and permits testing to proceed by straightforward manipulation of these devices by divers. However, such attempts have not met with substantial success, due largely to both the limitations of the underwater environment and the limitations on the abilities of divers to perform complex functions within that environment.
Ultrasonic inspection is a non-destructive testing and inspection method which beams a high frequency sound wave into the material being inspected, the reflections of such sound waves being used to detect surface flaws, subsurface flaws, thickness variations and other types of defects. In the straight beam technique, an ultrasonic probe is placed on one surface of the structure, which probe directs a beam of high frequency sound waves through the structure and listens for reflections of those waves. The sound waves travel through the material with some loss of energy, and are reflected at interfaces. The degree of reflection depends largely on the physical state of the material on the opposite side of the interface, and to a lesser extent on the specific physical properties of the two materials making up the interface. For example, sound waves are almost completely reflected at interfaces between metal and gas, but are only partially reflected at metal-liquid or metal-solid interfaces, and the degree of reflection depends somewhat on the properties of the materials on the opposing sides of the interface.
Ultrasonic inspection techniques provide ready detection of structural faults or discontinuities, including surface or internal cracks, laminations, pores, flaking, bonding faults, shrinkage cavities, and others. See, e.g., "Ultrasonic Inspection", in 11 Metals Handbook, pages 161 et seq. (8th ed. 1976). Inclusions of slag or other materials within the structure, even though such inclusions do not act as gas/metal interfaces, can also easily be detected by a variety of techniques, such as causing partial reflection or scattering of the ultrasonic waves, shear wave techniques, and other techniques known in the art.
Ultrasonic inspection devices function by transmitting ultrasonic mechanical vibrations, which impose stresses well below the elastic limit, through the structure being tested, and monitoring the time of transit of the transmitted wave, and reflections of such wave from inclusions, cracks, or the opposing surface of the structure. Many such devices also monitor attenuation or reduction in strength of the beam of sound waves being transmitted. Ultrasonic testing equipment commonly comprises an electronic signal generator that produces bursts of alternating voltage, a sending transducer or probe, which emits a beam of ultrasonic waves in response to the bursts of alternating voltage received from the signal generator, a couplant, which transfers the ultrasonic wave energy from the sending transducer to the test piece, a receiving transducer, to accept the output of ultrasonic waves which have traversed some portion of the test piece and convert such portion of the waves to corresponding bursts of alternating voltage, and devices for amplifying, viewing, and/or recording the signals received from the receiving transducer or probe. Quite often only one transducer is used for both sending and receiving, and that same transducer alternately transmits bursts of such ultrasonic waves and then listens for the response or reflection of those waves. In a typical case, the output of the probe will be amplified, etc., and reproduced visually, e.g. on an oscilloscope, wherein the response is plotted versus time. The thickness of the article being inspected can be gauged by the time transpiring between the transmittal of the ultrasonic signal and its reflections from the opposing surface or wall of the structure. Faults or non-uniformities within the structure appear as tips located between the peak on the visual display device which corresponds to the initial pulse of ultrasonic waves, and the peak which represents the reflection of those waves from the opposed surface of the structure.
As pointed out in the above-noted "Ultrasonic Inspection" article, this method of inspection has many advantages, including superior penetrating power, high sensitivity, and essentially instantaneous evaluation of the results. Also, ultrasonic testing requires availability of only one surface of the structure being tested, and is not hazardous, as compared, e.g., with radiation testing, to the personnel operating or surrounding the testing apparatus. However, as also pointed out in the above-mentioned "Ultrasonic Inspection" article, ultrasonic inspection suffers from the disadvantages that it requires careful attention to the instrument display by experienced technicians in order to obtain proper results, and extensive technical knowledge is necessary in order to interpret the results. Complex manipulation of the probe relative to the instrument display by such trained technicians is necessary in order to optimize readings, determine the size and shape of deformities or inclusions, and differentiate between the various types of faults which can occur, e.g. to differentiate between faults which will not worsen, such as lack of penetration of welds, and faults such as cracks, which may propagate and eventually cause failure of the structure.
The drawbacks inherent in the ultrasonic testing systems have largely curtailed their use in underwater experiments. Operation of such devices in a manner which permits successful detection of anything more than the most basic data, e.g. thickness, requires high skill both in the manipulation of the probe and in the adjustment of the highly complex instruments which display the output of the probe. The standard method for previous attempts at the use of such devices had been for the ultrasonic testing machine to stay on the surface, with a long cable attached to a transducer which a diver brings to the underwater surface to be checked for thickness. Communication between the diver and the technician operating the machine above is achieved through a headset in the diver's helmet. However, the technician operating the machine does not have the ability to move the transducer finite amounts to optimize the readings or determine the size or extent of faults. Nor does the technician/diver have the ability to guide his movements of the probe in conformity with the indications obtained on the ultrasonic testing machine.
As a result of these and other difficulties, no such instruments are available which permit underwater utilization of the ultrasonic technique for those functions in which it is most advantageous, e.g. fault, discontinuity, or inclusion inspection. Although some crude submersible ultrasonic instruments have been developed, they do not have the defect detection of standard ultrasonic testing devices, and are solely utilizable for gauging thickness. Further, all attempts to utilize the ultrasonic machine above the water level or topside, with the diver/technician manipulating the probe in response to orders from a topside technician have been unreliable and inconclusive, due to the inability of the topside technician to communicate the required "feel" to the diver below, to move the transducer various amounts in various directions to optimize readings and determine the size and nature of defects.
It is accordingly an object of the present invention to provide a system which permits utilization of ultrasonic testing for non-destructive underwater inspection and testing of structures, which gives reproducible and reliable analyses for a wide variety of structural defects, and which permits different techniques such as scattering and shear wave techniques, to be used in evaluating the underwater structure. It is a further object of the present invention to provide a system which permits the diver/technician operating an ultrasonic probe to obtain instantaneously the effect of movements of the probe on the article being tested. It is a further object of the present invention to provide a system for ultrasonic underwater testing which provides for immediate and reproducible permanent recordation of the ultrasonic readings obtained, to permit further evaluation after the physical inspection is completed.